THE ACTION OF >fEUTROJSfS ON THE RARE EARTH ELEMENTS 57 



9 h. period a second period, Jong and therefore not observed, is 

 present. 



Samarium also shows an absorption stronger than is to be expec- 

 ted from the activity of the known radiactive samarium isotope. 

 Of the numerous isotopes of samarium not leading to tlic formation of 

 active isotopes at least one must therefore have a strong resonance level 

 for slow neutrons. In view of the fairly weak activity of samarium the 

 absorption measurements could not be carried out by using a samarium 

 indicator, so rhodium was used for that purpose. The results of these 

 measurements and also of absorption measurements with other rare 

 earths using rhodium as indicator are shown in ta1)le 3. 



Table 3. — Percentage of Initial Intensity of the Neutron 

 Beam Present after the Passage of a "Thick" Layer 



It is well-known that the activity obtained by the action of slow- 

 neutrons is not a trustworthy measure of the intensity of the neutron 

 beam, because the neutron absorbing powers of different elements are 

 very specific and depend very much on the neutron velocities. The ambi- 

 guity arising from this fact can, however, be avoided by using the same 

 element as indicator and absorber in absorption experiments. Should 

 that not be feasible, as would happen if, for example, the absorbing 

 substance did not show any or had only a very slight activity — this is 

 the case with gadolinium — it is advisable to adopt the following pro- 

 cedure. The maximum absorption obtained in a thick layer of gadolinium 

 is measured using, say, rhodium as indicator ; then the thick layer is 

 replaced by a few milligrams of material and the absorbing power 

 measured again. The first mentioned measurement gives the result that 

 no more than 67% absorption can be obtained for the neutron beam 

 in question through a thick layer of gadolinium, while the last mentioned 

 measurement shows that2mgm of gadolinium are necessary to reduce the 

 intensity of the neutron beam by 10%. To arrive at a figure giving the 

 amount of gadolinium necessary to reduce the intensity of neutrons 

 of such velocities as are actually absorbed in gadolinium we must mul- 

 tiply 2 mgm by 0.67 and thus obtain a value of 1.3 mgm. The correspond- 

 ing figures for a few elements are given in Table 4 and 5. Of all the rare 

 earth elements gadolinium —as can be seen from the table — has the 



